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Spacecraft Angles
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This folder contains three files:

1. SpacecraftAngles_ReadMe.txt : the present file
2. SpacecraftAngles_v1.exe : a Windows executable program file
3. SpacecraftAngles_Unit.pas : the Pascal source code for this program

"SpacecraftAngles_v1.exe" is intended for use as a supplement to the Lunar Terminator Visualization Tool (LTVT) software. Its purpose is to estimate the camera location of a satellite photo for which only the camera's pointing angles and target point are supplied.

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BACKGROUND
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The most basic information needed to interpret a satellite photo of the Moon is the location of the camera, expressed in terms of the selenographic longitude and latitude over which it is located (the so-called spacecraft "nadir point"), and its radial distance from the Moon's center.  That information, together with the date and time and the coordinates of the lunar feature at the center of the field (the so-called photograph "principal point"), is required as input by LTVT. If it is a photo of a previously imaged area, the coordinates of the principal point and the rotational angle of the image can be inferred from the features portrayed, but the spacecraft position is difficult to infer. 

Unfortunately, the known spacecraft location is often suppressed in favor of supplying quantities derived from it, such as the tilt (relative to a line pointed towards the spacecraft nadir) of the camera axis, and its azimuth relative to a line pointing towards the Moon's north pole.  Sometimes the slant distance from the camera to the target point is also given.

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OPERATION
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"SpacecraftAngles_v1.exe" always requires the information in the center column:  the camera's tilt angle, azimuth and elevation above the lunar surface.  

1. Given the longitude and latitude of the spacecraft nadir point the software can compute the location on the Moon of the photo's principal point.  This is done by entering the nadir point in the input boxes on the left, and clicking the COMPUTE button on the right.  The answer will appear in the boxes above the  COMPUTE button.  This is mostly intended as a check on tabulated values when all the information is given.

2. More importantly for LTVT, given the coordinates of the principal point at which the spacecraft camera is pointed, the software can solve iteratively for the nadir point over which the camera must have been located to point at the specified principal point.  This is accomplished by entering the principal point longitude and latitude in the input boxes on the right and clicking the SOLVE button on the left. The result is displayed in the boxes next to the SOLVE button.  As a check on the calculation, the exact principal point expected from this nadir point and elevation with the specified camera angles is displayed on a line below.

The above calculations are all based on the assumption of a 1737.4 km radius spherical Moon.

For those who may be interested, the Pascal source code used for the computations is included in this folder.

-- Jim Mosher (jimmosher@yahoo.com)  19 June 2008
